1. Field of the Invention
The invention relates to a needle assembly, and more particularly, to a needle assembly with a collapsible sheath that can be retracted to permit observation of the needle tip prior to injection.
2. Description of the Prior Art
A needle cannula includes opposed proximal and distal ends and a lumen extending therebetween. The proximal end of the prior art needle cannula is mounted to a hub for placing the lumen in communication with a medical instrument, such as a hypodermic syringe. Typically, the distal end of the prior art needle cannula is beveled to define a sharply pointed tip. Health care workers and patients can be stuck accidentally with the distal tip of a needle cannula. An accidental stick occurring prior to use of the needle cannula is painful and can cause infection. An accidental stick occurring after use of a needle cannula can further transmit disease.
Most prior art needle cannulas are provided with shields that are intended to reduce the risk of accidental sticks. Some prior art shields include a resiliently collapsible sheath that surrounds the needle cannula prior to use. In use, these sheaths extend distally beyond the needle cannula, and consequently will contact the skin of the patient prior to making an injection. The sheath expands radially and collapses axially as the needle cannula is moved toward and into the patient. The sheath then resiliently returns to an axially extended, radially contracted condition as the needle cannula is withdrawn from the patient. An example of a prior art collapsible sheath for a needle cannula is shown in U.S. Pat. No. 4,139,009. Prior art needle cannulas with collapsible sheaths present a false sense of security. Health care workers may assume that the covered needle is safely protected. However, any proximally directed force on the distal end of the sheath will expose the needle cannula and create the potential for accidental needle sticks.
More recent prior art includes a collapsible sheath that is closely surrounded by a coil spring. The proximal end of the coil spring is fixed near the proximal end of the sheath. However, remaining portions of the coil spring can be collapsed in a proximal direction independent of the sheath. The sheath functions substantially as in the above described U.S. Pat. No. 4,139,009 when the coil spring is collapsed. However, the coil spring can be released to expand axially and to closely surround the sheath. The coil spring is intended to prevent the radial expansion of the sheath that is required to axially collapse the sheath. Hence, the coil spring is intended to keep the sheath in its extended position. This combination of a sheath closely surrounded by a coil spring is shown in U.S. Pat. No. 4,998,922.
Prior art needle cannulas with retractable sheaths closely engaged by coil springs have several disadvantages. For example, the tip of the needle cannula generally is not visible prior to making an injection. As a result, it is difficult to precisely target a specific location for an injection. Additionally, many injections require a particular orientation of the bevel on the distal tip of the needle cannula. The sheath, however, covers the distal tip and prevents precise orientation of the bevel. Furthermore, many injections require the needle cannula to be aligned at an acute angle to the skin. In this orientation, an edge region of the sheath will contact the skin, and will tend to slide abrasively across the skin as the needle cannula is being advanced. This abrasion unnecessarily adds to the discomfort imposed upon the patient.
Co-pending U.S. patent application Ser. No. 08/309,372 entitled "Needle Assembly with Collapsible Sheath and Tip Guard" was filed by the inventors herein concurrently with this application, and relates to a needle assembly with a collapsible sheath and tip guard. The disclosure of this co-pending application is incorporated herein by reference. Co-pending application Ser. No. 08/309,372 provides several advantages over the prior art disclosed in U.S. Pat. No. 4,139,009 and U.S. Pat. No. 4,998,922, including the provision of a tip guard for positively preventing accidental needle sticks. While serving to provide a device for safe, reliable re-exposure of the needle cannula during use, there exists a further need for a device which will allow a user to readily observe the distal tip of the needle cannula prior to making an injection.